Liquid streams used in or generated by industrial processes and drainage from mines, waste disposal sites and other industrial sites often contain dissolved heavy metals that are toxic and, if not removed, will be discharged into the environment causing serious pollution problems. Also, many of these heavy metals are of significant commercial value. A need exists to recover these heavy metals to conserve valuable metals and to prevent environmental pollution.
Heavy metals dissolved in waste and drainage liquids are often present in low, yet toxic, concentrations. Current techniques for recovering heavy metals at these low concentrations are either expensive or inefficient. Also, it is difficult to produce purified products of valuable heavy metals following heavy metal recovery techniques that do not allow for selective recovery of heavy metals. Selective recovery of heavy metals is desirable so that revenue from sale of valuable metals recovered can offset the cost of environmental clean-up of waste and drainage liquids and to conserve these valuable metals.
The present invention involves the use of xanthates to selectively recover dissolved heavy metals from liquid streams. Different affinities exhibited by different heavy metals for reacting with xanthated sawdust have been reported by Flynn et al., Absorption of Heavy Metal Ions by Xanthated Sawdust, Bureau of Mines Report of Investigating Actions No. 8427, U.S. Department of the Interior (1980). Solubilities of heavy metal xanthates in water are reported in Encyclopedia of Chemical Technology, edited by Kirk-Othmer, 3d Edition, Vol. 24, p. 648 (1981), Interscience Publisher, New York, N.Y.
Use of xanthates to recover dissolved heavy metals has been reported. Wing et al., Removal of Heavy Metals from Industrial Waste Waters Using Insoluble Starch Xanthate, Environmental Protection Technology Series, PB-283 792, U.S. Environmental Protection Agency (May 1978), discusses the use of an insoluble xanthated starch to recover heavy metals from waste waters generated by a printed circuit industry, lead battery manufacturers, and a brass mill. Flynn et al., Absorption of Heavy Metal Ions by Xanthated Sawdust, Bureau of Mines Report of Investigations No. 8427, U.S. Department of the Interior (1980), discusses the use of xanthated sawdust to recover dissolved heavy metals from dilute aqueous solutions, mine-drainage waters, and brines. In contrast to the present invention, however, these references, disclose the use of certain xanthates only as a means for nonselectively recovering dissolved heavy metals. These references do not disclose a process for selectively recovering dissolved heavy metals using xanthates. The value of many heavy metals can be realized only if selective recovery can be achieved. Also, nonselective recovery of all heavy metals is inefficient because some heavy metals may be efficiently removed by less expensive processes.